1. Field
Exemplary embodiments of the present invention relate to a semiconductor device, and more particularly, to a semiconductor device with a metal gate and a high-k materials and a method for fabricating the same.
2. Description of the Related Art
A CMOS device needs to lower the threshold voltages of N-channel and P-channel transistors, in order to perform a high-speed operation. N-type polysilicon has been used as a gate electrode of the N-channel transistor, and P-type polysilicon has been used as a gate electrode of the P-channel transistor.
However, with the decrease in size of transistors, the degradation of drive current is caused by depletion of polysilicon in the CMOS device. The depletion of polysilicon refers to a phenomenon in which the doping concentration of a dopant decreases at an interface with a gate dielectric layer. The dopant implanted in the polysilicon may be diffused out by a subsequent thermal process or the like. In this case, the doping concentration of the dopant decreases.
Therefore, the N-type polysilicon and the P-type polysilicon have a limitation in optimizing the threshold voltages of the respective transistors.
Recently, a transistor with a metal gate electrode, in which a metal is used as the material of a gate electrode, has been developed in correspondence to the miniaturization of transistors. A metal with a low work function is used for the N-channel transistor, and a metal with a high work function is used for the P-channel transistor. Here, the metal with a low work function is a material having a value corresponding to a work function (4.1 eV or less) of the N-type polysilicon and the metal with a high work function is a material having a value corresponding to a work function (4.7 eV or more) of the P-type polysilicon.
However, there is a limitation in adjusting the threshold voltages of the respective transistors to sufficiently low values, only through the method of adjusting the work functions of the metals. Furthermore, since a process for adjusting the work functions suitable for the N-channel transistor and the P-channel transistor is very complex, productivity inevitably decreases.